The present invention relates to a method for treating fumed silica in a continuous manner utilizing methyl substituted chlorosilane, aqueous hydrochloric acid and a surfactant.
Prior to the present invention, various methods were available for treating fumed silica to eliminate or reduce surface silanol groups on the surface of such silica. Experience has shown that untreated fumed silica or fumed silica having an average of about 1.9 to 5.3 silanol radicals/nm.sup.2 interacts with polydimethylsiloxane gum when used to reinforce such silicon gum resulting in crepe hardening. Crepe hardening is characterized by the compound being crumbly instead of elastic or soft. Crepe hardening is due to the formation of a network of polymer/filler particles that are loosely and reversibly "cross-link" to each other. This polymer-filler interaction is controlled by the hydrogen bonding between the silica surface silanols and both residual silanols in the silicon polymer and the oxygen of the polymer backbone itself.
Various procedures have been used to reduce the level of silanol on untreated fumed silica which can be in the form of the more reactive isolated silanols and both adjacent silanol and hydrogen bonded water. One procedure involves the batch treatment of the fumed silica over the period of several hours, such as 6 hours or more, with octamethylcyclotetrasiloxane. This treatment results in the production of fumed silica having surface silanol groups substantially eliminated.
Another procedure of treating silica filler is shown by A. Chuiko et al, U.S. Pat. No. 4,030,940, which shows silica filler treated with methylchlorosilane in the vapor phase and optionally in the presence of steam. An additional procedure for using fumed silica pretreated with dimethyldichlorosilane is shown by Matsushita et al, U.S. Pat. No. 4,340,090, directed to silicon compositions.
Although various procedures are available for treating silica filler treatment to enhance filler surface characteristics, improvements are constantly being sought. Particular emphasis is being directed to steps to reduce costs while maintaining improved filler characteristics. The present invention is based on the discovery that fumed silica can be treated economically in a continuous manner to achieve an optimum degree of surface pacification and thereby decrease interaction between polydimethylsiloxane gum and the filler by effecting contact between the fumed silica while in a fluidized state and at a temperature in excess of 400.degree. C., with a mixture of methylchlorosilane having the formula, EQU (CH.sub.3).sub.a SiCl.sub.4-a, (1)
water or hydrochloric acid, and surfactant as defined hereinafter, where a is an integer having a value of from 1 to 3 inclusive.